Semiconductor nanocrystals (also referred to as “quantum dots”) with small diameters can have properties intermediate between molecular and bulk forms of matter. Small diameter semiconductor materials can exhibit quantum confinement of both the electron and hole in three dimensions. Quantum confinement plays a key role in determining the size-dependent optical properties of semiconductor nanocrystals. One effect of quantum confinement is an increase in the effective band gap of the material with decreasing nanocrystal size. As the size of the semiconductor nanocrystal decreases, both the optical absorption and fluorescence emission of the nanocrystals shift to higher energy (i.e., to the blue). The extinction coefficient of the nanocrystal is also size-dependent. As the size of a nanocrystal increases, the extinction coefficient of the particle increases in a non-linear fashion. Consequently, for a given material, larger semiconductor nanocrystals are typically brighter than smaller nanocrystals on a per mole basis.
The quantum yield (QY) is an important measure for determining the quality of any given population of fluorescent semiconductor nanocrystals. A high quantum yield can indicate that some or all of these potential non-irradiative pathways are absent in the collection of quantum dots sampled. There are numerous mechanisms by which the fluorescence of a quantum dot can diminish or the quantum dot can become non-fluorescent (e.g., surface trap states, crystalline defects, photochemical effects, and the like). Many ways of improving the quantum yield of quantum dots have been proposed. In one approach to enhance quantum yield, nanocrystals can include a semiconductor core and a passivating layer. The passivating layer can be formed of organic compounds (also referred to as “ligands”) such as amines that coordinate to atoms in the core material. Yet other approaches utilize semiconductor shell materials with bandgaps higher than those of the core materials. Semiconductor shells can minimize deep-trap emission sites and thereby enhance the quantum yield and stability of the nanocrystal particle. Methods involving the use of organic ligands to improve the quantum yield in organic solution do little to enhance quantum yield in aqueous solution, while inorganic shells can improve the quantum yield in both organic and aqueous solution. Consequently, most recent advances in the field have moved in this direction. Despite the materials and methods available to prepare nanocrystals, there exists a need for new materials and methods to improve the organic and aqueous quantum yields of semiconductor nanocrystals.